Method for monitoring of tap changers by acoustic analysis

ABSTRACT

A method for monitoring an on-load tap changer wherein the recording of sound and its conversion into a sound signal are initiated at the beginning of a connection operation of the tap changer and the sound signal is rectified, converted into a signal envelope, sampled digitally and continuously evaluated with respect to the condition of the tap changer by comparing the envelope with at least one fixed or one continuously updated reference envelope. The comparison is carried out selectively by calculating a dimensionless comparison figure T i  which is based on a quantity measure and a variation measure.

TECHNICAL FIELD

The present invention relates to a method for monitoring of an on-loadtap changer of a power transformer or a reactor by evaluating the soundgenerated by the tap changer during the connection operation.

BACKGROUND OF THE INVENTION

Voltage control of power transformers and inductance control of reactorsused during electric power transmission are normally carried out withthe aid of an on-load tap changer which makes it possible to changebetween different terminals of a winding. This is to be done underloading, and for that reason the tap changer must be capable of handlingboth the voltage difference between the terminals and the currentthrough the winding. To reduce the intensity of the unavoidable arcswhich arise during the connection operation, usually also resistors areconnected during the connection operation. Depending on the location ofthe power transformer or the reactor in the electric network, the numberof connection operations of a tap changer during the technical life ofthe transformer or the reactor may amount to several hundred thousand. Atap changer has a complicated mechanical structure which, in combinationwith the great number of use cycles under mechanical loading, makes thetap changer one of the most exposed components of a power transformer ora reactor.

The tap changer is exposed to wear by mechanical abrasion and arcs. Thiscan manifest itself in a slow and gradual change of the performance ofthe tap changer; for example, the connection time may change because ofchanges in the dimensions and mutual distances of components included,such as contacts and other movable parts. The change process may at acertain stage accelerate and lead to the tap changer being incapable offunctioning. This may, among other things, cause costly power failures.

To prevent functional incapability of a tap changer, the tap changer ischecked. One way of doing this is an inspection of the tap changer,which requires opening of the transformer/reactor. This causes a lengthyservice interruption. This is therefore carried out as seldom aspossible, with an ensuing risk that a significant deterioration mayoccur before the next inspection.

It is known to continuously monitor the temperature of components of thetap changer, where the wear leads to an increased temperature, forexample in contacts which will have increased contact resistance becauseof carbon deposits originating from arcs in the transformer oil. Anincreased temperature is then a warning and alarm signal which givescause to inspection of the tap changer. However, significant heatingonly occurs in some of the change processes of a tap changer, and thenonly after an advanced ageing process.

Another known method is to continuously monitor the power consumption ofthe electric motor which drives the mechanics of the tap changer. Wearprocesses which manifest themselves in increased friction lead to anincreased power consumption during the connection operation which canthen be used as a warning or alarm signal. As in the case of temperaturemonitoring, this monitoring is limited to only some of the possible wearprocesses.

The connection operation generates sound which may be analyzed withrespect to changes in the tap changer. Changed distances, changedfriction etc., as a consequence of wear, influence the sound from theconnection operation. Japanese patent document A 6-13248 describes atechnique whereby the sound from a connection operation, which isdivided into a number of sound pulses related to part-processes, isanalyzed with respect to time differences between the beginning of thesepulses. If one or more of these time differences deviates too much fromvalues determined in advanced, a warning or alarm signal is released.U.S. Pat. No. 4,159,446 describes a system for detecting, by analogsignal processing of a signal envelope from a connection operation,incorrectly adjusted contacts by analysis of the signal envelope,preferably by a skilled person, with respect to time differences betweensignal peaks in the signal envelope. With the methods described inJapanese patent document A 6-13248 and in U.S. Pat No. 4,159,446, thereis a risk that not all changes in the tap changer manifest themselves inchanged time differences between the sound pulses and that only heavywear results in significant changes of these time differences.

A complete analysis of all sound from a tap changer at the time of eachconnection requires such calculating and memory resources that it is notpractically feasible.

SUMMARY OF THE INVENTION

A method according to the invention solves the problem of using thesound generated by a tap changer during the connection operation for acontinuous monitoring which detects gradual slow changes and rapidchanges in the sound picture, which are related to the status of the tapchanger without requiring the resources which would be needed for acomplete sound analysis.

When initiating a connection operation of the tap changer, a triggersignal is given which starts the recording of the sound as an electricalsignal, referred to in the following as a sound signal. The sound signalis rectified, converted into a signal envelope and sampled with arelatively low sampling frequency, typically 0.1-100 kHz. The signalenvelope contains the information necessary for monitoring the status ofthe tap changer within a data set which can be handled. The signalenvelope is permanently updated and is compared both with an envelopewhich is continuously adapted to the sound signal and with a fixedreference envelope, which may be the envelope for the sound signal upondelivery or after an inspection of the tap changer. The comparison ismade selectively at each sample point by forming a dimensionlesscomparison figure, which is based on quantity measure and variationmeasure which are related to some suitable statistical distribution. Ifthe comparison figure, for one or more sample points, exceeds apredetermined value, this leads to alarm or warning.

The conversion of the sampled and rectified sound signal into a signalenvelope preferably takes place in an analog circuit and thus requiresno additional calculating capacity. The envelope generation may bemathematically simply described as adding an exponentially decreasing"tail" to each sampled and rectified mean value and then replacing themeasured value by the maximum of the measured value and all thepreceding "tails". The advantage of this exponential envelope comparedwith other weight functions, for example a running mean over a number ofmeasured values, is that it retains the flank of steep signal changes,which is important for the characterization of the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical sound signal from a connection operation of anon-load tap changer.

FIG. 2 shows the sound signal from FIG. 1 after rectification andenvelope formation.

FIG. 3 shows a flow diagram for a method according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical sound signal from a connection operation of a tapchanger. It is a very unstable signal where a few major amplitude peakscan be distinguished. FIG. 2 shows the same signal after rectificationand envelope formation with a 10 ms time constant of the exponentiallydecreasing "tail". It is seen that sudden amplitude changes in theoriginal signal appear very clearly as upwardly moving steps of theenvelope.

A flow diagram for a method according to the invention is shown in FIG.3. The first steps in the signal processing are rectification, theenvelope formation already described above, and the removal of apossible offset in the signal.

For each sample number, i, of the envelope, counting from the start ofthe signal recording which is triggered during each connectionoperation, an arithmetic mean value M_(i) and a standard deviation s_(i)are calculated for the last N₀ recorded sample points with index i,where i means sample number counting from the start of the signalcollection during a connection operation. N₀ is selected according toexperience, a typical number being 10. If there are less than N₀ signalenvelopes, statistics is built up until N₀ connection operations haveoccurred; then the calculation is advanced one step after eachconnection operation, and the last envelope is included and the envelopewhich is N₀ +1 steps back in time is excluded. In this way, M_(i) ands_(i) are constantly updated and as running arithmetic mean value andstandard deviation they characterize the typical appearance of thesignal envelopes at the relevant time. The updating preferably occurswith a recursive algorithm which entails few calculation operations foreach step. Then there is formed, for each sample point with index i, adimensionless comparison figure T_(i) =| (S_(i) -M_(i))/s_(i) |which isthe deviation of the current sample value S_(i) with index i from themean value M_(i) with index i, normalized with the standard deviations_(i) with index i.

Instead of a standard deviation and an arithmetic mean value, which arerelated to the normal distribution, corresponding statistical parametersrelated to other distributions may be calculated to obtain a comparisonfigure, for example the mean value Ml_(i) and the standard deviationsl_(i) of the natural logarithms of the sample values, which logarithmsare related to the logarithmic normal distribution. In that case, alsothe current sample value S_(i) is replaced by its natural logarithmSl_(i) =ln(S_(i)).

It is suitable to redefine T_(i) as T_(i) =min (T_(x), T_(i)), whereT_(x) is an experience value, to reduce the influence of individualextreme sample values which may relate to temporary disturbances duringthe measurement. Typical values for T_(x) when using an arithmetic meanvalue are T_(x) =5 . . . 10. An additional measure for reducing theinfluence of occasional very deviating sample values is to not chooseT_(i) directly as a comparison figure; instead the running mean valueTm_(i) of the N_(t) last T_(i) is used, where N_(t) is chosen so as tocorrespond to 5-10 ms.

If at least one Tm_(i) for the current connection operation exceeds apredetermined value, for example within the range 4-5 starting from thearithmetic mean value when calculating T_(i), an alarm signal issupplied. The signal envelope has then changed significantly during thisconnection operation, which is a sign that a damage may have arisen inthe tap changer.

An alarm is released, as described above, at a sudden change of thesound signal envelope from a connection operation. In addition to suddenchanges, for example when some structural part loosens, which results inan alarm, a gradual change of the sound signal envelope also occurs. Itmay be related to a corresponding gradual change of the tap changer, forexample mutual distance changes through wear. Such gradual changes aredetected by continuously comparing the updated mean signal envelope witha reference signal envelope, which may, for example, be taken up whenputting the tap changer into service or after an inspection and anyrepair thereof. This is done, for example, by using the samecalculations as during an alarm by replacing the sample S_(i) of thesignal envelope by a reference mean value Mr_(i) for each index i. If atleast one corresponding Tm_(i) exceeds a predetermined value, forexample the same as during an alarm, a warning signal is given, which isa sign that a significant gradual change of the status of the tapchanger may have occurred.

Instead of distinguishing between alarm and not alarm and warning andnot warning, respectively, both alarm and warning may be graded,depending on the magnitude of T_(i). For example, an alarm or a warningat a highest level may provide a reason for putting the tap changerimmediately out of service and immediately opening the powertransformer/reactor, whereas an alarm or a warning at a lowest levelbecomes one indication among others for judging a suitable time forplanned maintenance of the tap changer.

What is claimed is:
 1. A method for monitoring an on-load tap changer,comprising:initiating recording of sound and conversion of the soundinto a sound signal at a beginning of a connection operation of the tapchanger; rectifying the sound signal; converting the sound signal into asignal envelope; digitally sampling the sound signal; continuouslyevaluating the sound signal with respect to a condition of the tapchanger by comparing the envelope with at least one fixed or onecontinuously updated reference envelope, wherein the comparison isselectively carried out by calculating, at each sample point with indexI, a dimensionless comparison figure T_(i) that is based on a quantitymeasure and a variation measure, where the quantity measure and thevariation measure relate to a statistical distribution; and releasing analarm or a warning when the comparison figure T_(i) for one or moresample points exceeds a predetermined value.
 2. The method according toclaim 1, wherein the dimensionless comparison figure T_(i) is calculatedaccording to T_(i) =|(S_(i) -M_(i))/s_(i) |, where S_(i) is a currentsample with index i, M_(i) is a arithmetic mean value with index i ands_(i) is the standard deviation with index i for a last N₀ recordedsample values.
 3. The method according to claim 1, wherein thedimensionless comparison figure T_(i) is calculated according to T_(i)=|(Sl_(i) -Ml_(i))/sl_(i) |, where Sl_(i) is the natural logarithm of acurrent sample with index i, Ml_(i) is a arithmetic mean value withindex i and sl_(i) is a standard deviation with index i for logarithmsof a last N₀ sample values.
 4. A method according to claim 3, whereinthe dimensionless comparison figure T_(i) is limited by an upper limitvalue T_(x) according to T_(i) =min (T_(i), T_(x)).
 5. The methodaccording to claim 2, wherein in that a running mean value Tm_(i) isformed from the last number N_(t) of dimensional comparison figuresT_(i) and that this running mean value Tm_(i) is used as a dimensionlesscomparison figure.
 6. The method according to claim 5, wherein in thatthe number N_(t) is chosen so as to correspond to a time between 3 and20 milliseconds.
 7. The method according to claim 2, wherein the numberof N₀ lies between 3 and
 30. 8. The method according to claim 1, whereina sampling frequency of the signal envelope lies between 0.1 and 100kHz.